U.S. Pat. No. 5,366,630 discloses a method of controlling a dialysis apparatus for hemofiltration treatment by establishing the positive transmembrane pressure (TMP) between first and second compartments of a dialyser having two compartments separated by a semi permeable membrane, the first compartment being connected to a blood circuit for conveying blood outside of a human body and the second compartment having an inlet connectable to dialysate liquid circuit. The value of the TMP is measured and compared with a threshold value. When the measured value is greater than the threshold value treatment liquid is supplied to the inlet of the second compartment the exchanger and effectively the treatment mode is switched from hemofiltration to hemodiafiltration.
The Gambro AK 200 ULTRA™ online dialysis machine can be controlled in a number of different modes of operation of which the two most relevant modes will be discussed here. Before the treatment is started, the desired weight loss of the patient is determined, and is divided by the treatment time in order to get a desired weight loss rate. In a first mode of operation, usually referred to as “volume control”, the desired total replacement volume is divided by a treatment time to get the infusion rate. The pump for the replacement fluid is set to deliver fluid at this desired rate. The fluid balancing system will compensate for the amount of replacement fluid. A certain TMP will result, depending on the characteristics of the membrane and the blood composition. Thus, a predetermined amount of fluid is ultrafiltered to the dialyser and the correct weight loss of the patient is reached at the end of the treatment time. In another mode of operation, usually referred to as “pressure control”, a desired TMP is set in order to achieve a high ultrafiltration rate higher than the desired weight loss rate. Often this TMP set point is chosen close to the maximum TMP allowed. The chosen TMP will resolve in a certain ultrafiltration rate, which will depend on the blood composition and characteristics, i.e. permeability of the chosen dialyser. The resulting ultrafiltration will determine the infusion flow rate that has to be produced by the apparatus. The treatment is stopped when the accumulated ultrafiltration liquid volume reaches the predetermined value.
These two methods for control can each under certain circumstances produce alarms because some threshold values are exceeded. In order to shorten the length of the treatment in volume control, it is desirable to set the infusion rate as high as possible. It is however difficult to know exactly the limits set by the filtering capacity of the dialyser. In addition, as the treatment progresses and the blood becomes thicker due to the accumulating weight loss, this filtering capacity will gradually decrease. The result is frequently that the infusion rate is set at a value which may work at the start, but as the treatment progresses there is an increasing number of TMP alarms due to insufficient filtering capacity.
There is a similar situation in pressure control. If the TMP is set too close to the maximum allowable TMP, the resulting ultrafiltration rate may become so high that the necessary replacement fluid rate cannot be achieved by the pump. This will create an alarm situation.